1. Field of the Invention
The invention relates to a fuel cell and a gasket for a fuel cell.
2. Description of the Related Art
As one of the measures to address environmental and resource-related issues, fuel cells, which generate electric power by converting chemical energy into electric energy through an electrochemical reaction using oxidizing gas, such as oxygen and air, and reducing gas (i.e. fuel gas), such as hydrogen and methane or liquid fuel, such as methanol, etc., have drawn much attention. The fuel cell has been considered and examined from various aspects with expectation that the fuel cell can be an environmentally clean energy source, based on the facts that gases and liquid fuel, which are source material used for generating electric power, are plentifully available, and that only water is produced and discharged from the fuel cell because of the principle of power generation employed in the fuel cell.
A unit fuel cell (i.e. unit cell) includes a membrane electrode assembly (MEA) that is sandwiched by separators, such as metal separators. The MEA includes a fuel electrode (i.e. anode catalyst layer) on one surface of an electrolyte membrane, and an air electrode (i.e. cathode catalyst layer) on the other surface of the electrolyte membrane, so that the fuel electrode and the air electrode are disposed on the opposite sides of the electrolyte membrane. A plurality of unit cells are stacked to form a fuel cell stack. Each separator has fluid passages. More specifically, fuel gas passages and oxidizing gas passages are formed on the surface of the separator facing the MEA. Coolant passages are formed on the surface of the separator opposite to the surface facing the MEA. The fuel gas passages, oxidizing gas passages, and coolant passages are formed in an area of the separator that corresponds to the area of the MEA where electric power is generated (hereinafter simply referred to as “power generation area”). Further, fuel gas manifolds, oxidizing gas manifolds, and coolant gas manifolds are formed in an area of the separator where electric power is not generated (hereinafter simply referred to as “non-power generation area”). Fuel gas flows through the fuel gas manifolds and the fuel gas passages, and oxidizing gas flows through the oxidizing gas manifolds and the oxidizing gas passages. Coolant flows through the coolant manifolds and the coolant passages. These fluid passages are sealed from outside by providing sealing members, such as adhesive agent or gaskets, around the area where the fluid passages are provided. In the two unit cells disposed adjacent to each other, a seal between the separators of the unit cells is provided by a sealing member, such as an adhesive agent or a gasket. For example, Japanese Patent Application Publications No. 2004-165125 (JP-A-2004-165125) and No. 2004-146282 (JP-A-2004-146282) describe that a gasket is provided between two metal separators to seal therebetween.
As described in JP-A-2004-165125 and JP-A-2004-146282, when a rubber gasket is employed as a sealing member used in a fuel cell, adhesion and firm fixation of the rubber gasket can lead to reducing sealability, which is originally the feature of the gasket. For example, when the fuel cell is started at a low temperature, such as at a temperature below the freezing point, the fuel cell stack is thermally expanded, and thus the gasket should behave in compliance with the thermal expansion of the fuel cell stack. However, if the gasket sticks to the separator, compliance of the gasket is reduced. When resistance to pressure should be secured in a steady state, “sealability” is determined based on both of “linear sealing pressure on the gasket” and “adhesion”. Thus, the sealability becomes better as the adhesion becomes higher. On the other hand, in a dynamic state, for example, when the fuel cell stack is thermally expanded, if the gasket sticks to the separator at the point where the gasket contacts the separator, deformation of the gasket is restricted. Due to this restriction in deformation, there is a possibility that the cells are locally bent, and weak adhesion portions of the plural cells are deformed due to such local bending, thereby locally creating openings between cells. Therefore, leakage of fluids may occur.